I am wondering if anyone knows how to create an Xcode OS X (.app) application that could control an Arduino via USB. Basically, I want to replace Processing with Xcode, and create a Mac application that controls my Arduino. Thank you.
The Arduino is a virtual serial device. So your X-Code (Cocoa, not really "X-Code") application will need to open a serial port. Beyond that, there are plenty of resources out there on how to program for OS X.